The basic criteria for lighting design include illuminance, brightness, uniformity (lowest illuminance/average illuminance), coefficient of utilization (the flux received in the effective luminance range/the lighting source flux), luminaire efficacy (luminaire flux/light source flux), and so on. There is a trade-off between the coefficient of utilization and uniformity. It is a big challenge to improve high coefficient of utilization while to maintain the uniformity. How to reach a good balance between the coefficient of utilization and the uniformity remains a big task to the lighting designer.
Recently, the LED lighting is becoming popular. As the LED lighting has the advantages of eco-friendliness, high efficiency, low maintenance cost and long lifespan, the LED lighting will replace the conventional lighting source eventually, such as mercury lamp, incandescent lamp, halogen lamp. Since the single LED's flux is not sufficient for the luminance needed, an LED array with plurality of LEDs is needed. This type of LED light source has the following drawbacks:    1. Different lighted targets may require different second-order optical designs according to the distance from the light source (such as different height of the road), the shape of the lighted area, or the lighted space (different road width or distance between lamps). The suitable lighting distribution curve cannot be achieved by simply changing the LED array arrangement.    2. The LED light source usually uses a lamp casing as the second-order optical reflector; hence, it is difficult to form optimal radiation pattern.    3. LED's light radiation is directional, thus, the LED light source can easily generate glare and cause uniformity problem which makes the user uncomfortable.    4. The same LED chips may generate different radiation patterns because of the different packaging manner or packaged by different manufacturers. Therefore, the second-order optical design of the lighting device is restricted by the packaging manufacturer and the packaging method.
Therefore, the present invention provides an optical module which can guide the LED light radiation to the righted area with expected efficacy.